1. Field of the Invention
The present invention relates to a demodulation method and apparatus therefor. More particularly, this invention pertains to the demodulation of simultaneous transmissions from multiple sources such as that encountered when navigating with Global Positioning System (GPS) data.
2. Description of the Prior Art
GPS is a navigation system that includes a plurality of space vehicles (satellites) that transmit radio frequency signals to earth. Each space vehicle's location at any given point in time can be determined with precision by calculation from ephemeris or almanac data. Distances and velocities with respect to such locations can be determined for navigation purposes from the transit times and rate of change of transit times of signals originating with the various satellites. When signals are simultaneously received from at least four satellites, locations and directions on (or below or above) earth can be determined by trilateration methods.
The need to process signals originating from multiple sources complicates the design of GPS-type system receivers. Some designs have featured separate receivers and/or separate dedicated channels in a single receiver to perform the multiple signal demodulation. Such approaches incur significant costs while increasing system power, weight and/or bulk requirements greatly.
Attempts have been made to solve the above problems by multiplexing among the independent signals within a single hardware channel. Such an approach is disclosed in "An Advanced NAVSTAR GPS Multiplex Receiver" by Phil Ward, IEEE Journal pp. 51 (1980) and in U.S. patent Ser. No. 4,468,793 of Johnson et al. for "Global Position System (GPS) Multiplexed Receiver". In the system disclosed in that patent, signals from a plurality of satellites are sequentially sampled one time during each transmitted data period.
While the above-described method does result in cost, bulk and power savings, such method is not sufficiently broadband for many applications. Further, the narrowband operation that is achieved by such a system is particularly sensitive to the loss of signal lock that can occur when an earth vehicle (or and airplane or ship) performs relatively "high g" maneuvers.